FIG. 1 depicts a schematic diagram of a telecommunications system in the prior art. Telecommunications system 100 comprises:                i. telecommunications network 101,        ii. network telephone 103,        iii. private branch exchange 105,        iv. on-premises telephones 106 and 107,        v. Internet protocol network 108,        vi Internet protocol telephone 109,        vii. cellular protocol network 110, and        viii. cellular telephones 112 and 113,all of which are interconnected as shown.        
Telecommunications network 101 is one of multiple, telecommunications networks that are represented in FIG. 1. Telecommunications network 101 comprises the Public Switched Telephone Network, which is a complex of telecommunications equipment that is owned and operated by different entities throughout the World. In the United States of America, for example, the Public Switched Telephone Network (or “PSTN”) comprises an address space that is defined by ten digits, and, therefore, comprises 10 billion unique addresses or “telephone numbers.” The public switched telephone networks in other countries are similar.
Network 101 interconnects the other telecommunications networks that include (i) the enterprise network supported by private branch exchange 105, (ii) Internet protocol network 108, and (iii) cellular protocol network 110. The enterprise network supported by private branch exchange 105 provides telecommunications service to one or more telecommunications terminals—for example, telephones 106 and 107—within the enterprise area served, such as an office building or campus. Internet protocol network 108 provides telecommunications service to one or more Internet protocol-capable telecommunications terminals, including telephone 109. Cellular protocol network 110 provides telecommunications service to one or more cellular telecommunications terminals, including telephones 112 and 113.
Additionally, network 101 provides telecommunications service to other telecommunications terminals, such as telephone 103. For example, network 101 is able to route a call that telephone 103 originates to private branch exchange 105.
Internet protocol network 108 is capable of switching incoming calls from network 101 to terminals that are capable of Voice over Internet Protocol (VoIP), such as telephone 109. Network 108 is also capable of handling outgoing calls from VoIP-capable terminals to network 101. Network 108 handles calls that involve telephone 109 via one or more routers.
Cellular protocol network 110 is capable of switching incoming calls from network 101 to cellular-capable terminals that have registered with the network, such as telephones 112 and 113. Network 110 is also capable of handling outgoing calls from cellular-capable terminals to network 101. Network 110 handles calls that involve telephones 112 and 113 via one or more mobile switching centers and radio base stations.
Private branch exchange (PBX) 105 is an enterprise system capable of switching incoming calls (e.g., originated by telephone 113, etc.) from telecommunications network 101 via one or more communications paths to one or more on-premises terminals, such as on-premises telephones 106 and 107. Private branch exchange 105 is also capable of handling outgoing calls from on-premises terminals to network 101 via one or more communications paths.
Private branch exchange 105 is also capable of forwarding an incoming call, such as from telephone 113, to a telephone number of a PBX user's “off-premises” terminal that is accessible through network 101. This type of forwarding to a terminal affiliated with exchange 105 is also known as “extending” a call because the connection to the off-premises terminal appears to exchange 105 as an additional PBX line, or “call appearance.” Exchange 105 extends the call to the call appearance at the off-premises terminal in addition to switching the same incoming call to a call appearance at an “on-premises terminal” within the enterprise area that exchange 105 serves. In telecommunications system 100, telephones 106 and 107 are the on-premises terminals with respect to private branch exchange 105, while telephones 103 and 112 are the off-premises terminals with respect to exchange 105. Note that in system 100, telephones 113 and 109 are not considered off-premises terminals because, unlike telephones 103 and 112, they are not affiliated with exchange 105.
To accomplish (i) the switching of an incoming, enterprise-related call to an on-premises terminal and (ii) the extending of the call to the correct off-premises terminal, private branch exchange 105 maintains a table that correlates the off-premises telephone number to the on-premises, private branch exchange extension. Table 1 depicts a table that illustrates the correlation.
TABLE 1PBX Extension-to-PSTN Number DatabaseOn-PremisesPrivate BranchOff-PremisesTelecommunicationsTelephoneExchange ExtensionTelephoneNetwork Number106732-555-0102, x11103201-555-1236107732-555-0102, x12112908-555-3381. . .. . .. . .
In addition, private branch exchange 105 is capable of receiving an incoming call attempt from an off-premises terminal, such as from telephone 103 or 112, in which the call attempt is for accessing one or more PBX user features. Note that outside of the enterprise network, only those terminals that are affiliated with exchange 105 (i.e., telephones 103 and 112) are intended to have access to the user features. In handling the call attempt, network 101 passes to exchange 105 the caller identifier that represents the calling, off-premises terminal. Exchange 105 checks that the caller identifier matches with one that is stored (e.g., 908-555-3381, etc.) and, as a result, grants the PBX user at the off-premises terminal the privilege to access one or more features.
The problem with exchange 105 relying on the caller identifier of the calling terminal is that a telephone user who is using a bogus telephone can masquerade as the legitimate PBX user by manipulating (i) the bogus telephone or (ii) the calling network, or both, to represent the bogus telephone as the legitimate one. This was not a major concern when telecommunications networks were somewhat closed systems that were not easily compromised. However, with the advent of broadband telephone services (e.g., Vonage, AT&T CallVantage, etc.) that coexist with the Internet, it has become easier for hackers to manipulate telecommunications networks for unethical or unlawful purposes, such as to “spoof” the caller identifier of a legitimate calling party. For example, a user of a VoIP telephone in certain broadband telephone networks, such as telephone 109 in Internet protocol network 108, is able to use a three-way calling feature to manipulate the value of the VoIP telephone's caller identifier. If the user of a bogus telephone takes on the number of a legitimate user of exchange 105, the bogus user becomes able to access features that are reserved only for legitimate, off-premises users. Access by illegitimate users enables breaking into voice mail accounts and the calling of legitimate PBX users for gathering proprietary information. To minimize this unwanted activity, what is needed is a way to detect the spoofing of a telephone number, without some of the disadvantages of the prior art.